Non-linear distortion in receiver's analog front end is characterized by its non-linear transfer function. The overall system non-linear transfer function is described in FIG. 1.
The non-linear transfer function can be modeled, in its nearly linear region 10, with the following model:Y(X)=A1·X+A2·X2+A3·X3+ . . .
Where A1 is the linear gain 12 of the system, A2 is the second order non-linear coefficient and the A3 is the third order non-linear coefficient. Higher order powers of X are negligible in this region in terms of their contribution to the distortion of the signal passing through the non-linear system.
Generally the second and third order distortions are characterized by an intercept point, which indicates the output power of the non-distorted signal 22,28 when equal in power to the non-linear distortion product 24,30. For example, the third order intercept point 26, denoted by IP3, indicates the output power of the non-distorted signal 28 when equal in power to the third order intermodulation 30. This output power point is never to be attained.
The third order intermodulation 30, characterized by the input power and the IP3 26 and the corresponding second order intermodulation 24 characterized by the input power and IP2 20 are described in FIGS. 2b and 2a, respectively.
The intermodulation product concept of IP3 26 and IP2 20 has been generally developed for 2-tone sine wave input scenarios. However the non-linear transfer function also applies to both multiple sine wave input and wideband signals. Using the 2 tone input signal model, the relation between the IP2 20 and IP3 26 and the non-linear coefficients A2· and A3 can be derived yielding the following relations:
      A    1    =                    10                  (                      Gain            /            20                    )                    ⁢                          ⁢              A        2              =                            -                                                    A                1                4                                            2                ·                                  IP                  2                                ·                                  Z                  in                                                                    ⁢                                  ⁢                  A          3                    =              -                              A            1            3                                3            ·                          IP              3                        ·                          Z              in                                          
Where,                Gain—is the system linear gain in dB.        IP2—is the system output 2nd order intercept point        IP3—is the system output 3rd order intercept point        Zin—is the system impedance        
The intermodulation distortion products of 2 sine wave tones in frequencies f1 and f2 are located at frequencies m·f1±n·f2 where the |m|+|n| defines the order of the distortion, e.g. for third order distortion the intermodulation products are at frequencies 2·f1−f2, 2·f2−f1, 2·f1+f2 and 2·f2+f1. This means that generally second order distortion can be filtered since the distortion frequencies are located outside the signal while third order distortion can not be filtered in a conventional way, especially in wideband signal case, since the distortion may be located inside the signal bandwidth. An example for a distorted wideband signal is shown in FIG. 3 where a wideband distortion 32 is centered at 3 times the signal center frequency and a wideband distortion 34 is on the original signal.